Surge arresters are used to protect equipment connected to power distribution networks from damage by excessive voltage situations caused by lightning strikes, switching surges, incorrect connections, and other abnormal conditions or malfunctions.
The active element in a surge arrester is often a varistor, also referred to as a non-linear varistor because it exhibits a non-linear current-voltage relationship. If the applied voltage is less than a certain voltage (the switching or clamping voltage), the varistor is essentially an insulator and only a small leakage current flows through it. If the applied voltage is greater than the switching voltage, the varistor's resistance drops, allowing an increased current to flow through it. That is, a varistor is highly resistive below its switching voltage and substantially conductive above it.
The surge arrester is commonly attached to an electrical power system in parallel configuration, with one terminal of the device connected to a phase conductor of the electrical power system and the other terminal to ground or neutral. At normal system voltages, the surge arrester is resistant to current flow (except for the leakage current). However, if an overvoltage condition exceeding the switching voltage develops, the surge arrester becomes conductive and shunts the surge energy to ground while “clamping” or limiting the system voltage to a value which can be tolerated without damage to the equipment being protected.